1. Field of the Invention
The present invention relates to a phase-locked loop circuit and, more, particularly, to a loop filter that is used therein.
2. Description of the Related Art
FIG. 2 is a block diagram of a typical conventional phase-locked loop circuit composed of a phase comparator 1 which performs detection of a phase difference, a loop filter 2, and a voltage-controlled oscillator 3. An input signal is supplied to one input of comparator 1 via a line 4 and an output signal is produced by oscillator 3 on ]ine 5. The signal on line 5 is fed back to the phase comparator 1, thus forming a phase-locked loop circuit. Comparator 1 produces a phase advance signal 6 and a phase delay signal 7 which are supplied to filter 2. Loop filter 2 generates an output signal on line 10 which is input to voltage-controlled oscillator 3 as an oscillation control signal.
Upon detecting a phase difference between the signals on lines 4 and 5, comparator 1 generates either the signal 6 or 7 to control the voltage-controlled oscillator 3 through the loop filter 2 in a direction to reduce a phase difference.
FIG. 3 shows one example of a loop filter used in conventional phase-locked loop circuits. The illustrated loop filter includes a charging source constituted by a combination of constant-current sources 8 and 9 each producing driving current I.sub.O, switches SW1 and SW2 and a low-pass filter which is constituted by a resistor R with a resistance value R.sub.O, and a capacitor C with a capacitance value C.sub.O. Switches SW1 and SW2 are preferably semiconductor devices.
Assuming that a leading phase difference with a time duration tl is detected, the phase advance signal 6 is produced so that SW1 turns ON. As a result, the voltage level of the output signal on line 10 of loop filter 2 instantaneously rises by I.sub.O .times.R.sub.O) and further a voltage rise corresponding to the amount by which capacitor C is charged by curent source 8 is gradually added to output signal on line 10. The voltage rise reaches (I.sub.O .times.t.sub.1)/C.sub.O after the time t.sub.1. When the phase difference becomes zero, SW1 is turned OFF, so that the voltage held across the caparitor C is outputted as the output on line 10 of loop filter 2.
Conversely, when the phase delay signal 7 is produced SW2 turns ON. As a result, a voltage decrease (I.sub.O .times.R.sub.O) is caused and the charge held in capacitor C is also gradually removed, so that the output signal 10 of loop filter 2 continuously decreases as long as a lagging phase difference is detected. When the phase difference becomes zero, SW2 turns OFF, and the voltage across the capacitor C is outputted again as the output signal on line 10 of loop filter 2.
Thus, loop filter 2 may be regarded as a phase-to-voltage converter which outputs a voltage corresponding to a phase difference detected by the phase comparator. In accordance with the converted voltage output, the voltage-controlled oscillator is controlled such that a phase difference detected is corrected continuously.
The above-described conventional phase-locked loop circuit suffers, however from the following problems. The first problem is that it is difficult to fabricate the conventional phase-locked loop circuit in the form of an integrated circuit. More specifically, since the conventional loop filter includes a low-pass filter which comprises a resistor R and a capacitor C, if the resistance value R varies, the change in voltage (I.sub.O .times.R.sub.O) also varies as a matter of course. Accordingly, it is necessary in order to obtain stable loop filter characteristics to minimize variations in the resistance value of resistor R.
On the other hand, the capacitance value that is employed for a loop filter fabricated in the form of an integrated circuit is limited to several tens of pF because of limitations on area. Accordingly, I.sub.O must be reduced in dependence on C.sub.O in order to fabricate a loop filter having the conventional arrangement in the form of an integrated circuit without altering the rate of change of the voltage level (I.sub.O .times.t.sub.1)/C.sub.O as the capacitor C is charged or discharged for the time t.sub.1 with the constant current I.sub.O in response to a detected phase difference. At the same time, when I.sub.O is reduced, the instantaneous change in voltage (I.sub.O .times.R.sub.O) would also decrease; therefore, R.sub.O must be increased in order to achieve the desired instantarneous voltage change.
More specifically, let us consider a phase-locked loop circuit the transfer function H(S) of which is expressed as follows: ##EQU1##
In expression (1), Wn is the natural angular frequency, S is the Laplace c,perator and .xi. is the damping coefficient. Wn and .xi. are expressed as follows: ##EQU2##
In these expressions, C and R denote the respective values of the capacitor and resistor which constitute in combination the loop filter, while K denotes the closed-loop gain of the phase-locked loop circuit. The closed-loop gain K is given by the product of the charge-and-discharge current I of the constant-current sources constituting a charge pump and the conversion coefficient Kv of the voltage-controlled oscillator as follows: EQU K=I.times.Kv (4)
To design a phase-locked loop circuit, it is general practice to determine optimal values for Wn and .xi. and then select constants for the loop filter in conformity with the determined values. For example, in the case of the phase-locked loop circuit of a data separator which is employed in a floppy disk drive, Wn and .xi. are selected as shown telow so that the output of the circuit will not follow a jitter (peak shift) of a read data input: EQU Wn=30.times.10.sup.3 [rad/s]
.xi.=0.7
Let us obtain loop filter constants with which the above-described values for Wn and .xi. are obtained. Assuming that the conversion coefficient Kv of the voltage-controlled oscillator is 75.times.10.sup.3 [H.sub.Z /v] and C is 50pF, K is obtained from the expression (2) as follows:
K=CWn.sup.2
=0.045
Accordingly, the current source current I is obtained from equation (4) as follows:
I=K/Kv
=0.6[.mu.A]
On the other hand, the resistance R is obtained from the expression (3)as follows: ##EQU3##
Thus, a considerably high resistance value is calculated.
As has been described above, the resistance R of the loop filter when fabricated in the form of an integrated circuit must have a high precision and yet a high resistance value. Examples of resistors which may be used in integrated circuits include diffusion resistors and polycrystalline resistors. The former type of resistor has a relatively high resistance and is therefore advantageous from the viewpoint of area but it exhibits large variations in the resistance value. In the case of the latter type cf resistor, on the other hand, the resistance value accuracy is high, but it is disadvantageous from the viewoint of area. Accordingly, in either case, it is difficult to fabricate a phase-locked loop circuit in the form of an integrated circuit without a change in the arrangement of the conventional loop filter.
The second problem of the prior art is concerned with a lock-in oper.ation that is performed at the time when the power supply is turned ON. Immediately after the power supply has been turned ON, voltage across the capacitor C of the loop filter is zero and, therefore, the oscillation control signal on line 10 applied to the voltage-controlled oscillator 3 in the arrangement shown in FIG. 2 is also at the zero level. Accordingly, at the time when the power supply is turned ON, the frequency of the voltage-controlled oscillator 3 differs from the so-called lock frequency (center frequency) by a substantial amount, so that it takes a long time to reach a locked state from the deviated state. If the gain of the loop filter is increased in order to reduce the lock-in time, it becomes easy for the output of the loop filter to follow input jitters after a locked state has been reached; therefore, it is not preferable to increase the gain of the loop filter. If the sensitivity to jitters is lowered, it takes a relatively long time to effect lock-in and, in an extreme case, it is impossible to reach a normal locked state.
It has also been proposed to manufacture such loop filters as analog, or nonintegrated, devices external to an integrated circuit containing the other phase-locked loop components. However, it is inconvenient to connect such filter to the integrated circuit and to accurately adjust the operating parameters of the analog filter.